In general, an electrode assembly having a step has a structure as illustrated in FIG. 1. An electrode assembly 1 having such a step includes an anode or a cathode disposed on both surfaces thereof, as outermost electrodes in a thickness direction of the electrode assembly, that is, a stacking direction thereof.
An electrode assembly having a step as described above is formed by arraying electrodes 7, such as cathode electrodes or anode electrodes, having a difference in areas thereof on a separation film 20 elongated in a single direction, in a predetermined sequence, or by arraying units cells 5having different areas, formed by stacking a cathode and an anode on each other to have a separation film 20 serving as a boundary therebetween, in a predetermined order, and then folding the separation film to correspond to an electrode unit or unit cell as shown in FIG. 2. Here, the separation film is stretched tight to closely adhere to an electrode surface of a respective electrode or unit cell so as to be flattened thereon, such that the separation film encompasses a previously stacked electrode laminate to continuously allow subsequent electrodes or unit cells to be stacked on an upper surface or a lower surface of the electrode laminate.
Therefore, a step portion having a step formed due to a difference in areas between an electrode having a relatively large area and an electrode having a relatively small area is provided with an inclined surface 10 formed thereon, without closely adhering to side portions formed by electrode laminates stacked on each other, to encompass the previously stacked electrode laminates, as illustrated in FIG. 3.
In the case as described above, a step portion between respective layers may have tension generated in the separation film when an electrode assembly is assembled, and such separation film tension may degrade insertion properties at the time of inserting an electrode assembly into a battery case. In addition, tension generated in the separation film may lead to a defect in a shape of an electrode assembly due to bending occurring in a lower electrode having a large area, as shown in FIG. 4. Accordingly, in an electrode assembly assembled using a stacked and folded scheme, tension generated in a separation film as described above is required to be removed.